Multimedia services become increasingly important not only in wireline environments, but also in wireless environments. The multimedia services in both wireline and wireless environments should offer customers a high quality experience.
One important feature to provide such kind of experience is Quality of Service (QoS) provision and reservation. QoS provision and reservation ensures that the services provided and requested in a service providing network can be transmitted with the respective resources they require. In this context, QoS refers to resource reservation control mechanisms. For example, by reservation of a required QoS, a required bit rate, delay, jitter, packet dropping probability and/or bit error rate may be guaranteed for a respective service.
In the IP Multimedia Subsystem (IMS), the node controlling resource reservations is the Policy and Charging Rules Function (PCRF). Within IMS the PCRF is the policy entity that forms the linkage between the service and transport layers. The PCRF collates subscriber and application data, authorizes QoS resources, and instructs the transport plane on how to proceed with the underlying data traffic. The PCRF is connected on its northbound Rx interface to the Application Function (AF), an element residing on the service plane, which represents applications that require dynamic policy and QoS control over the traffic plane behavior. Within an IMS network, a Proxy-Call Session Control Function (P-CSCF) would commonly fulfill the role of an AF. The P-CSCF is a Session Initiation Protocol (SIP) proxy that is the first point of contact for an IMS terminal.
The PCRF receives information about offer/answer exchanges between terminals from the AF over the Rx interface. If the characteristics of the session being established are acceptable to the PCRF (based on the domain policy), the PCRF authorizes the session on the access gateway using a Gx interface. If the characteristics of the session are not acceptable to the PCRF, it instructs the AF to terminate the session using the Rx interface. Of course, in this case the PCRF does not authorize the session on the access gateway.
When the PCRF informs the AF that the session being established is not acceptable, the AF must inform the terminal trying to establish the session. This information from the AF to the terminal would normally not carry an indication of the type of session or the time of session set up that would be acceptable to the network. Therefore, it would be difficult for the terminal to generate a new acceptable offer. Additionally, the user experience would be far from ideal. The session would be established and terminated immediately. Thus, a terminal is not always informed why its session was rejected. The user of the terminal may not understand what to do to establish a new session that will be accepted. A user may also receive a successful-session establishment indication and a session-terminated indication, one immediately after the other.
In short, by means of the P-CSCF and the PCRF, for any kind of session (voice, video, audio, conversational, non-conversational), a resource reservation is made via the Rx interface. Depending on the kind of service, a certain QoS class is requested and if resources are free, this class is granted. This kind of resource reservation, however, always takes place at session set up, which may lead to decreased usability.
Compared to just in time services, scheduled services like podcasting are gaining more importance. In these scheduled services, however, time constraints for media delivery exist. These time constraints normally vary dependent on the type of the scheduled service. As an example, podcast services should usually be delivered to a user requesting the podcast in a time range of one or more hours, like one to twelve hours, e.g. 6 hours. Streaming services, however, may require a shorter time range, like a range of up to several minutes. Thus, the above procedure is not well suited for scheduled services that operate according to strict conditions relating to scheduling.
In current deployments, there is no mechanism for prioritization of services. Resource reservation always takes place upon session set up, regardless of the kind of service. There are cases, though, where it is known beforehand that certain resources are going to be needed at a certain time. In the case of e.g. podcast-like media delivery, all subscribers to the service will get the media items at the same time, usually when updated content becomes available. This kind of peak phenomenon can easily lead to network congestion, if no measures are taken.